1. Field of the Invention
The present invention relates to an image processing apparatus that performs image processing, and more particularly, to an image processing apparatus that distinguishes image attributes, particularly halftone-dot regions, and performs image processing appropriate for such attributes.
2. Description of the Related Art
In the conventional art, when image processing apparatuses such as printers perform printing or other processing of halftone-dot regions, the phenomenon of moiré can occur. As a result, the occurrence of moiré has been prevented by extracting halftone-dot regions from the image data and carrying out smoothing regarding the extracted halftone-dot regions. As a process for extracting halftone-dot regions from image data, a process has been proposed wherein the image data is divided into blocks having a prescribed range and it is determined whether the characteristics of each block correspond to those of a halftone-dot region (Japanese Laid-Open Patent Application 2002-27242).
However, the following problem exists in connection with the conventional image processing apparatus described above. Namely, the image data may include figures that are determined to be isolated points, depending on the character configuration. In particular, in the case of small-sized characters (particularly characters that are 5-point or smaller), areas bordered by lines can be detected as white isolated points. Furthermore, the dot in the letter ‘i’ or the small lines at the bottom in such characters as the Japanese character  may be detected as black isolated points. In addition, a dot formed by the intersection of lines may also be detected as a black isolated point. A region in which these characters are concentrated may be incorrectly determined to be a halftone-dot region even if it is not. Furthermore, because smoothing is carried out to such erroneously determined regions, the sharpness of the characters contained therein may deteriorate.
Specifically, in a halftone-dot region in which isolated points are distributed evenly as shown in FIG. 7, 12 isolated points are extracted from the block shown in FIG. 7. On the other hand, in a character region, three of the characters  in a small point size may be concentrated in a single block, as shown in FIG. 8. In this situation, four white isolated points are extracted for each character . Consequently, from this character region, a total of 12 isolated points are extracted from the three characters. As a result, when the determination of whether or not a halftone-dot region exists is based on the number of isolated points, because the block in FIG. 8 contains the same number of isolated points as the halftone-dot region in FIG. 7, it is erroneously determined to be a halftone-dot region even though it is in fact a character region.